storage: api refactor.

Here's the big ol' refactor taking over from #194.

- Docs: http://stephenplusplus.github.io/gcloud-node/#/docs/master/storage
- Tree: https://github.com/stephenplusplus/gcloud-node/tree/spp--storage-refactor/lib/storage
- Test tree: https://github.com/stephenplusplus/gcloud-node/tree/spp--storage-refactor/test/storage
- Regression tests: https://github.com/stephenplusplus/gcloud-node/blob/spp--storage-refactor/regression/storage.js